1. Field of the Invention
The invention relates to electrodes for measurement of ion activity in solution and, more particularly, to low-maintenance reference electrodes for such purposes.
2. Background Information
The measurement of ion activity in solution has wide-ranging application in both science and industry. In medicine, the concentration of various ions in the blood and other body fluids can be an important indicator of a patient's health. In industry, knowledge of the presence or absence or various ion species, and their concentrations, may be critical to various processes. In examining water quality, ion concentration measurements are often critical to assessing the viability of water resources.
Electrode-based ion measurement systems typically comprise a measuring electrode (often referred to as a sensing or test electrode) which is responsive to the particular ion whose concentration in solution is to be measured and a reference electrode which provides a stable junction potential against which the measuring electrode potential is to be compared. The sensing electrode typically is connected to the solution being tested by an ion-selective material that is preferential for the ion to be measured; the reference electrode is typically connected to the solution by an electrically conductive “bridge” or “junction” through which electrically conductive material (“filling solution” or “reference solution”) travels to form a circuit for measuring current flow. These junctions take a variety of forms.
One common form of junction is a porous material such as a ceramic frit which allows an electrically conductive internal reference solution to seep out from the electrode and into the solution being tested to thereby establish an electrically conductive path between the electrode and the solution. An example of such a junction is shown in U.S. Pat. No. 4,495,052, issued Jan. 22, 1985 to Brezinski. Such junctions are susceptible to fouling and thus must periodically be removed from service for cleaning or even replacement.
Another form of junction is shown in U.S. Pat. No. 3,915,829 issued Oct. 28, 1975 to Krebs which discloses a polycarbonate sheet having a multiplicity of sub-micron channels formed in it by ion bombardment. This type of electrode relies on a continuous flow of electrolyte reference solution through the junction and into the test solution to prevent fouling. This, of course, eventually depletes the reference solution. A similar form of junction is described in U.S. Pat. No. 7,344,627 issued Mar. 18, 2008 to Broadley et al. (as well as earlier patents in this same patent family) which disclose a polycarbonate sheet having a multiplicity of nano-sized channels formed in it by ion bombardment. A positive pressure is applied to the electrolyte reference solution in order to establish a flow of sufficient intensity to prevent fouling.
Still another form of junction is the so-called “Hamilton pH sensor” which uses a single channel or pore to connect the internal reference solution to the test solution. To the same effect see also U.S. Pat. No. 4,959,138 to Brinkmann et al.